The present invention relates to cable television systems or the like, and more particularly to a set top converter that is compatible with both analog monaural and NICAM digital stereo audio.
The NICAM audio system uses two audio carriers. One of the carriers contains an FM modulated analog monaural audio signal. The other is a QPSK (quadrature phase shift keyed) carrier containing a compressed digital stereo audio signal. As an example, the analog FM sound carrier frequency can be 5.5 MHz, with the digital sound carrier at 5.85 MHz. In another example, the analog and digital sound carriers can reside at 6.0 MHz and 6.552 MHz, respectively. A specification describing the NICAM system in detail can be found in EBU Technical Recommendation SPB 424, Revised Version, "Digital Sound Transmission in Terrestrial Television," European Broadcasting Union, Brussels, Belgium, Apr. 1987.
The broadcasting of FM radio signals over cable television networks has never achieved significant success due to poor signal quality and the difficulty in collecting revenue by controlling access to the service. New digital techniques for the reproduction of sound provide performance that is far superior to analog techniques which have been used in the past. An example of high fidelity sound reproduction using digital techniques can be found in the compact disc technology which has recently enjoyed tremendous success as an alternative to phonograph records and tapes. Digital recording and playback techniques provide reproduction of music that is extremely realistic and free from background noise and distortions that have plagued other high fidelity sound reproduction systems currently in wide scale use.
In the United States, the Broadcast Television Systems Committee (BTSC) of the Electronic Industries Association has selected a television stereo audio system to serve as the standard for broadcast television. The format selected consists of a multiple subcarrier scheme developed by the Zenith Electronics Corporation and a noise reduction companding system developed by the DBX Corporation. A completely different system, known as NICAM, has been adopted for the transmission of stereo television sound in various countries outside of the United States. As noted above, the NICAM audio system uses two separate carriers for the original FM modulated analog monaural audio and the compressed digital stereo audio.
Cable television system operators in countries using the NICAM transmission format desire to provide NICAM digital stereo audio services to their customers. Typically, such cable systems transmit signals to a subscriber's television or other video appliance via a set top coverter that is connected between the cable drop and the receiving appliance. Baseband processing is advantageous in a cable television converter because it allows for enhanced scrambling of both audio and video signals. Baseband processing also enables the provision of volume control through the set top converter. The complexity of the NICAM signal, however, renders it prohibitively expensive to demodulate, volume control, and then remodulate in a baseband converter. Therefore, it would be advantageous to provide a baseband converter that is compatible with the NICAM audio system. Such a converter should enable the use of enhanced scrambling techniques available via baseband processing. The ability to adjust an audio signal's volume should also be provided via the converter.
The present invention provides a NICAM compatible converter having the aforementioned advantages.